1. Field of the Invention
The present invention relates to an optical interconnect module that transmits optical signals at high speed through a transmission channel or path over a relatively short distance. More particularly, the invention relates to an optical multiplexing interconnect module that multiplexes a plurality of signals and distributes them through an optical transmission channel.
2. Description of the Related Art
In recent years, electric interconnection has reached a limit of practical use. This is because the performance of electronic devices, such as bipolar transistors and field-effect transistors, has increased very much. It is proposed that electric interconnections be replaced by optical interconnections that optically connect LSIs or printed circuit boards. Any optical interconnect module is advantageous in some respects. First, it uses a signal transmission channel that exhibits virtually no frequency-dependency and causes no loss, over a range from direct current to tens of GHz. Second, it can help achieve high-speed transmission, since it is free from electromagnetic interference and makes no noise resulting from ground-potential changes.
Takai et al., “800 Mbit/s/ch×12ch, True DC-Coupled Parallel Optical Interconnects Using Single-Mode Fiber and 1310 nm LD Array,” 2000 Electronic Components and Technology Conference, discloses an “optical interconnect module in which signals are transmitted in parallel through optical transmission channels or channels.”
In the optical interconnect module disclosed in the Takai et al. thesis, a signal representing several Gbps of data can be allocated to each of the optical transmission channels that are available. Hence, the module can distribute data as much as several Gbps times the number of the channels available. Assume that 32 optical transmission channels are available, each being able to transmit 5 Gbps of data. Then, the module can provide optical interconnections for such a great amount of data as 160 Gbps (i.e., 20 GBps (B: byte).
In any optical interconnect module, a certain optical transmission channel is allocated to the clock signal CLK. The data used in most digital LSIs is composed of so-called “NRZ (Non Return to Zero) signals, each being a clock-signal wave that has a logic value of “0” at the valley and a logic value of “1” at the peak. That is, the clock wave is a signal that has a one-clock period. To transmit the clock waves, it is therefore necessary to use a transmission band that is about twice as broad as the data-channel band.
An optical interconnect module, in which each optical transmission channel can transmit, for example, 5 Gbps at most, needs a transmission band of about 3.5 GHz in order to transmit a digital signal wave. This is because the data signals have fundamental frequency of 2.5 GHz. In contrast, the clock signal has fundamental frequency of 5 GHz. Hence, in an optical interconnect module, which transmits the clock signal and a digital signal wave at the same time, the clock-signal channel must have twice as broad a band. To this end, two clock signals of different phases, each having twice as long a period, are transferred through two channels and are then synthesized.
The clock signal may not be transferred, for the purpose of use the interconnect channels as effectively as possible. In this case, however, a clock-extracting circuit must be employed. Further, the clock-extracting circuit can hardly extract the clock signal from the NRZ signal in some cases (for example, when the data is long-period one, i.e., “11111 . . . ”). Thus, it is desirable to transfer the clock signal to accomplish reliable signal interconnection. In the conventional optical interconnect module, the band for the data-signal channels must therefore be limited to the narrow band for the clock-signal channel.
The conventional optical interconnect module must have many optical interconnect channels to transmit a great amount of data, inevitably because it uses as many optical interconnect channels as the data signals to transmit. Therefore, the optical transmission channel may become too broad or may lack flexibility in some cases. Further, the operating band of the optical elements and electronic circuits incorporated in the module limits the signal-transmitting band of each optical channel.
Thus, the conventional optical interconnect module is disadvantageous in several aspects. First, the band for the data-signal channels must therefore be limited to the narrow band for the clock-signal channel. Second, the optical transmission channel is too broad or lacks flexibility. Third, the operating band of the optical elements and electronic circuits limits the signal-transmitting band of each optical channel.